Silicon is a material that is increasingly used for the fabrication of mechanical parts and particularly micromechanical parts, whether they are “captive” parts, i.e. which remain connected to a substrate on which they have been machined, or “free” parts, such as parts forming part of the kinematic chain of a timepiece movement.
Compared to the metals or alloys conventionally used for fabricating micromechanical parts, such as toothed wheels, hinged parts, or springs, silicon has the advantage of having a density which is three to four times lower and thus of having very reduced inertia, and of being insensitive to magnetic fields. These advantages are particularly advantageous within the field of horology, both as regards isochronism and the duration of operation.
Silicon has, however, the justifiable reputation of being sensitive to shocks, which may be necessary during assembly, inevitable during operation, or accidental for example when the user knocks the wristwatch against something or drops it.
WO Patent document No. 2007/000271 proposes a method for improving the mechanical resistance of silicon micromechanical parts. According to this prior art document, once the parts have been micro-machined in a silicon wafer, the surface thereof is coated with a layer of silicon dioxide. This layer is formed by thermal oxidation of the surface of the part at a temperature of between 900° C. and 1200° C. The duration of the thermal oxidation process is determined so as to obtain a thickness of silicon dioxide which is at least five times greater than the thickness of native silicon dioxide.
One drawback of the method which has just been described is that the silicon dioxide substantially increases the friction coefficients of the parts obtained. To overcome this drawback, the aforementioned document proposes adding a coating of a different material onto the amorphous silicon dioxide. The different material must be selected for its tribological properties. This solution, which consists in adding an additional coating on top of the silicon dioxide layer, has the drawback of multiplying the layers of coating. Moreover, certain coatings do not adhere well to the silicon dioxide.